A review of the “health debt form”, under which patients are charged up to 300 by a GP for documentation to prove they have mental health issues

Addressing the Charity Commission, Mrs May will say: “For too long mental illness has been something of a hidden injustice in our country, shrouded in a completely unacceptable stigma and dangerously disregarded as a secondary issue to physical health.

“Yet left unaddressed, it destroys lives, it separates people from each other and deepens the divisions within our society. Changing this goes right to the heart of our humanity; to the heart of the kind of country we are, the values we share, the attitudes we hold and our determination to come together and support each other.”

The speech follows Mrs May’s announcement at the weekend that she wishes to create a “shared society”, with the state taking a greater role in ending “unfairness”.

She will say: “This is a historic opportunity to right a wrong, and give people deserving of compassion and support the attention and treatment they deserve. And for all of us to change the way we view mental illness so that striving to improve mental wellbeing is seen as just as natural, positive and good as striving to improve our physical wellbeing.”

Mind chief executive Mr Farmer said it was “important to see the prime minister talking about mental health”.

He added: “The proof will be in the difference it makes to the day-to-day experience of the one in four who will experience a mental health problem this year. Mental health is everyone’s business and we need to see sustained leadership to make sure services and support improve”.

“Having been neglected for decades, we need to see it made a priority for decades to come to make sure everyone with… problems can live the life they want to lead.”

Mrs May’s emphasis on a “shared society” marks a contrast with her predecessor David Cameron’s “Big Society” agenda, which relied on voluntary organisations rather than state intervention.